An ion cyclotron uses a fixed magnetic field to deflect an ion moving at some velocity through the field. For a spatially uniform magnetic field having a flux density B, a moving ion of mass m and charge q will be bent into a circular path in a plane perpendicular to the magnetic field at an angular frequency .omega..sub.o in accordance with: .omega..sub.o =qB/m. Thus, if the magnetic field strength is known, by measuring the ion cyclotron frequency it is possible in principle to determine the ionic charge-to-mass ratio q/m. In effect, the static magnetic field converts ionic mass into a frequency analog. Because the cyclotron frequencies for singly charged ions (12.ltoreq.m/q.ltoreq.5000) in a magnetic field of about 3 Tesla span a radio frequency range (10 kHz.ltoreq.f.ltoreq.4 MHz), within which frequency can be measured with high precision, the ion cyclotron is potentially capable of offering extremely high mass resolution.
In an ion cyclotron cell, the ions may be formed by irradiation of a neutral gas or solid by various known techniques, including the application of electron, ion, or laser beams directed along the magnetic field. The ions are trapped in the cell because the static magnetic field constrains the ions from escaping anywhere in a plane perpendicular to the field and a small static trapping voltage is applied to the end plates of the cell to prevent the ions from escaping in a direction parallel to the field. However, even ions having the same mass-to-charge ratio and the same initial velocity are created at random points in time, and therefore with random phase, i.e., having random angular positions in their circular paths. These incoherently moving ions cannot produce a detectable signal in the cell. To detect the ions, it is necessary to apply an oscillating electric field in a direction normal to the magnetic field. This radio frequency electric field drives those ions having a natural cyclotron frequency equal to that of the electric field continuously outward in their orbits, whereas those ions in the cell whose natural frequency is not near to the frequency of the applied field do not resonate with the field and thus are not driven to larger orbits.
Various techniques have been used to detect the resonant ion cyclotron motion. One technique, as used in the omegatron type ion cyclotron resonance mass spectrometer, measures the current produced as ions continuously spiral outward into a detector plate. Another technique measures the power absorbed by the resonant ions from the exciting electric field. Such techniques generally rely on excitation of the cell with an oscillating electric field at a single frequency, with the frequency of oscillation being changed from time to time to scan over the desired frequency range or the magnetic field being varied to bring ions of various charge to mass ratios into resonance at a fixed frequency. Single frequency techniques were found to be badly limited with respect to both mass resolution and the time required to gather a mass spectrum. Significant increases in resolution and speed have been obtained using Fourier transform techniques wherein the whole spectrum is excited at once and the whole spectrum is thereafter detected at once. Such Fourier transform ion cyclotron resonance spectroscopy techniques are described further in the patent to Comisarow and Marshall, U.S. Pat. No. 3,937,955, the disclosure of which is incorporated herein by reference.
Since introduction of Fourier transform ion cyclotron resonance (FTICR) mass spectrometers, significant progress has been made in improving the detection of the resonant ions--for example: by reducing the base pressure in the cells, extending the bandwidth of the detection circuit, shielding of the transmitter and detector leads and using a differentially pumped dual cell. The swept frequency pulse excitation described in detail in the foregoing Comisarow, et al. patent, with detection taking place after the excitation is turned off, has been implemented for wideband excitation and is still the primary excitation used in the various presently available FTICR instruments. Ideally, such excitation signals have an essentially flat excitation power over a band of frequencies of interest to excite ions of various mass-to-charge ratios to a common orbital radius, thereby yielding a mass spectrum in which the intensities of the various peaks accurately reflect the relative numbers of ions having the mass-to-charge ratio values at which the peaks are located. Although the frequency sweep excitation has a relatively flat power spectrum compared to other types of Fourier transform excitations commonly used--e.g., single pulse excitation and pseudorandom noise--substantial variation in the excitation power spectrum is observed; that is, the spectrum is not perfectly flat but varies cyclically and substantially as a function of frequency and has relatively broad excitation band shoulders. In addition, the frequency swept excitation necessarily excites all frequencies between the lowest and highest frequency in the excitation band, and thereby does not allow selected large peaks in the ICR response to be suppressed to enhance the detectability of nearby small peaks. Such broad band excitations also cannot be used to eject ions of all but one mass-to-charge value, nor can the excitation be used to detect several specified mass-to-charge values simultaneously while suppressing detection of non-selected mass-to-charge values which may lie between the selected values.